Group by feature

[binaryk]

Adding a way to group by so the frontend could ask:

/api/restify/posts?group_by=status

[cristijora]

Things to consider if implemented

• Should be added to filter endpoints
• All group by fields should also be sortable
• When we group_by=status, it should also sort=status because we want to group all the the things that are matching the same statuses in this case.
• In most cases you'd want to group by a related entity e.g group_by=creator.name for example.